Question

seja a P.G (-1, x, -9,...) qual é a soma dos oito primeiros termos​

Answer 1

$\large\boxed{\begin{array}{l}\rm PG(-1,x,-9,\dotsc)\\\rm a_3=a_1\cdot q^2\\\rm -9=-1\cdot q^2\\\rm q^2=9\\\rm q=\sqrt{9}\\\rm q=3\\\rm S_8=\dfrac{ a_1\cdot(q^n-1)}{q-1}\\\\\rm S_8=\dfrac{-1\cdot(3^8-1)}{3-1}\\\\\rm S_8=\dfrac{-1\cdot6560}{2}\\\\\rm S_8=-3280\end{array}}$